The sine wave step-down converter, also known as a step-down travel platooninsert, is a sine wave AC/AC converter, where the function of step-down and stable voltage and frequency can be realized in the AC/AC transformation process. At present, most AC/AC portable devices in the market output modified waves, where the voltage is firstly rectified to a DC voltage by a step-down circuit, secondly stepped down by the BUCK and finally inverted to AC voltage to be provided for a load. However, such step-down converter mostly outputs a voltage presented as a modified wave, which is quite harmful to electrical devices, and meanwhile has a low efficiency and a large volume that is inconvenient for carrying.